It has long been recognized that exercise with weights can be highly beneficial for most people. Historically, the principal obstacle to its practice has been the problem of keeping the weights under control while they are being lifted. The danger involved with placing oneself beneath a heavy weight, with only one's own strength and skill to prevent the weight from falling, is daunting even for experienced weightlifters. Professionals in the field continually caution that heavy weights should not be lifted unless others are standing by to help in an emergency. Even when weights do not fall, the threat of their doing so can cause injury. Unnatural movements, made to keep a weight from going out of control, can easily cause strains in the already heavily loaded muscles. Many injuries to muscles of the back have occurred in this way. Even in the absence of injury, the value of weightlifting is always compromised by the need to make one's first concern the control of the elevated weights, leaving the exercise of particular muscles to whatever capabilities remain.
Over the years, recognition of these deficiencies has inspired the creation of a variety of devices intended to make this kind of exercise safer and more effective. These have generally fallen into two broad categories: weight guidance systems and weight substitution systems.
Weight guidance systems usually only allow vertical movement. In modern designs, the weights are usually separated from the means upon which the one exercising exerts effort. The connection between the two is most often made by cable and pulley arrangements, though some relatively inexpensive ones currently available use a rigid mechanical connection. The weights move vertically in a guideway. A central structure, which in the better systems is usually quite bulky, supports the various components.
In almost all of the apparatus of this type currently available, the primary motion available to the one exercising is in a vertical plane, in the upward direction. The downward return to the starting position is accomplished by the impetus of the weights themselves. Some designs may be converted to a downward vertical movement, with an upwards return. The ease with which this can be done is usually in direct proportion to the cost.
Weight guidance systems are beneficial, but they represent only a partial solution to the problem of control. Limiting degrees of freedom reduces the number of potential causes of accidents, but the degree of freedom which must remain, the vertical, has by far the greatest potential for harm. In all of these systems, it is still possible for the actuation means to be driven back into the user, impelled by the weights to which it is attached. This danger of "backlash" has caused designers in the field to make undesirable compromises to assure a reasonable degree of safety. The most common of these is a severe limitation on the travel of the operating means, combined with a particular shaping of the means itself. Typically, the operating means is located relatively high on the central structure, with its travel restricted mechanically to about two (2) feet. This tends to assure that in the case of a backlash, the user is unlikely to be trapped beneath the operating means. The design of the operating means itself is usually complementary in this purpose. Instead of being configured as a horizontal bar, the most ergometrically desirable shape to give something which must be pushed or pulled vertically, these known means usually have the shape of handlebars. The virtue of this is that in a backlash situation, the two handles are likely to pass around the user. Its principal disadvantage is an operational one. Handlebars require that the user's hands be placed in prescribed locations, regardless of how awkward or uncomfortable it may be for a particular individual. The combination of this and restricted movements limits the usefulness of most of these devices.
Attempts to overcome problems such as these have brought about the existing family of weight substitution apparatus. These deal with the problem of backlash by eliminating the weights which cause it. The ones currently available use dynamic reactions to create the forces which the user must overcome. The most common approach is to have a hydraulic or air cylinder connected to an operating means, which moves the piston inside. Movement of the piston forces the fluid in the cylinder through an orifice. The pressure necessary to do this is translated into a force on the piston, which is then seen as a resistance to movement of the operating means. These devices are basically "passive" in that their operating means move only in response to an urge from the user. They can still produce a backlash due to residual pressure, but the extent and severity of this is usually minimal. Because of this, they commonly have operating means featuring true horizontal bars. Their principal disadvantages are a difficulty in making precise adjustments of resistance, and a pronounced sensitivity to speed of movement. It is a basic characteristic of the principles by which these machines operate that resistance to movement increases drastically with the speed of the movement. Through rigorous design, these problems can be minimized but the solutions are difficult and expensive to implement.
This category also contains a type of apparatus known as a clutch/flywheel device. In this, the operating means moves a cable, which turns a drum to which is attached a clutch and flywheel assembly. Resistance is attained through acceleration of the mass of the flywheel. The clutch engages the flywheel only when the operating means is moved. This type of device is also sensitive to speed of movement. This concept is inherently free of backlash.
Most of the existing devices in the "weight substitute" category are mechanically quite simple, with the result that their inherent idiosyncracies of performance remain intact. Because of this, their use is usually restricted to instances in which safety is of the greatest importance.
For a great many years, inventors have considered the idea of using electro-magnetically developed resistance as the basis of an exercise machine. See, for example, Gardiner, U.S. Pat. No. 444,881, dated 1891, and Raymond, U.S. Pat. No. 670,006, dated 1901. Each consisted of a small generator which was turned by two cables which were to be pulled by the user's hands. In both of these devices, the current so generated was to be transmitted through the cables to the body of the user. It seems that at that time, it was widely believed that passing a small amount of current through the body was beneficial. Fortunately, neither of these would have been capable of producing much power.
The generator based system is also shown in Cooper, U.S. Pat. No. 857,447, dated 1907, which described a "rowing machine". The operating means was a hand held bar having cords attached to the respective ends. The other ends of the cords were wound around a shaft, which was connected to a generator. When the cords were pulled, the generator turned. Rewind was by automatic reversal of the generator into a motor. This was clearly an attempt at a "passive" system, although it seems to have been a largely unsuccessful one. Its control arrangement was not capable of effectively varying its resistance, and weights had to be included for that purpose.
The problem of control is common to all generator based resistance systems. Under constant excitement, the torque requirements of generators vary drastically with speed of rotation. The relationship is direct, but nonlinear, and difficult to control precisely over a wide range of speeds, without recourse to electronic power supplies run by computers. Even with this, it is effectively impossible to maintain normal running torque resistances into the low speed range. This characteristic inevitably leads to difficulties in exercise apparatus, in which low speeds are a normal part of the operational regime.
Attempts have been made to overcome these problems, but success has been limited. In 1975, Flavell, in U.S. Pat. No. 3,869,121, described a generator based apparatus which featured a sophisticated electronic control system to maintain a constant level of resistance. Even this would have been ineffective at low speeds, so a speed increasing means was used to minimize the time spent in this operating region as well as to reduce the size of the generator required.
Limitations similar to those of generators also apply to eddy current brakes. From some points of view, these can be seen as crude generators which dissipate their electrical output internally. Their torque resistance also varies nonlinearly with speed of rotation, and can be controlled in the same ways. They are less expensive than generators of comparable torque resistance, and energy absorbing capability, but require more excitement current, so what is saved in the cost of machinery is lost again in the cost of its power supply. The search prior to this application disclosed one such device, patented in the Soviet Union (SU No. 0869781) in 1981.
The concept of a brake as the source of resistance of an exercise device is an attractive one. Brakes are inherently "passive", being incapable of moving under their own volition. As a group, they are generally simple, relatively inexpensive, and well adapted to the task of energy dissipation. Ordinary friction brakes, however, have characteristics which limit their usefulness in this application. Among them are problems with breakaway torque, fade, wear, and controllability under rapidly changing load condition. There are attractive alternatives though. The general category into which the eddy current brake fits contains two other types which have characteristics which are well suited to this application. These are the magnetic particle brake and the magnetic hysteresis brake. Both use low power magnetic fields to develop a resistance to motion.
The magnetic particle brake contains a magnetically permeable powder such as iron or mild steel, between its rotor and stator. Electromagnetic coils in the stator magnetize the powder, causing it to bridge between the stator and the rotor, and, in doing so, develop resistance to motion.
The magnetic hysteresis brake uses the well known principal from which it gets its name to develop a resistance to torque. Typically, the stator is circular and formed into an annular shape. The rotor is a hollow cylinder, coaxial with the stator, and supported at only one end, the other rotating within the stator's annulus. The stator contains a series of magnetic pole structures. These are disposed radially, the ones in the outer part of the stator facing inward, and the ones in the inner part facing outward. The poles alternate between north and south, the series running continuously around the circumference of both the outer and inner portions of the stator. The poles are energized by electrical coils in the stator.
When the coils are energized, a magnetic field fills the annulus, its intensity and polarity at any given point depending upon the polarity of the coils nearby, and the degree of excitement of the coils. The rotor is made of an easily magnetized material, and it acquires a pattern of magnetic charges from the field in which it is immersed. As the rotor is turned, its pattern of impressed charges will change to match its changing orientation in the magnetic field, but because of hysteresis, there will always be a difference between the field and the pattern of charges in the rotor. The result is a series of attraction/repulsion reactions between the rotor and the stator, which produce a resistance to movement of the rotor.
Both of these types of brake typically develop torque resistances, which vary only slightly with speed of rotation. Additionally, "breakout torque" (the torque necessary to initiate rotation from a stopped condition) of each is normally less than 5% greater than the running torque. Commercially available examples of each type feature "constant excitement" torques which vary less than plus or minus 5% from 0 to 5000 rpm. This level of performance is achieved with a very simple control system. Typically, all that is required is a low powered, variable voltage, DC power supply. Voltage is variable for the purpose of varying torque resistance in the brake. Normally, the precision of torque resistance adjustment is equal to that of the power supply, with the provision that at a "no excitement" condition, some residual mechanical friction is always present. With this as a minimum, existing commercial examples of both offer a range of adjustment of better than 20:1.
Of the two, the magnetic particle brake is better suited to use in exercise apparatus. For any given level of physical size and expense, it produces an order of magnitude greater torque than the hysteresis brake. Although both have been commercially available for more than 20 years, a patent search has revealed only one attempt at an application in exercise apparatus, European patent application No. 81304852.7, filed 1981 by one A. C. Bently, residing in Rossmore, Calif., concerning the invention of one F. J. Bruder, of Newport Beach, Calif. The apparatus described is a rudimentary device, in which a magnetic particle brake is turned by a simple crank attached to its rotor. The application also describes a relatively unsophisticated control system intended to vary the brake's resistance according to the position of the crank.
In general, the field of electrically based exercise apparatus in the "weight substitute" category seems to be characterized by an emphasis on innovation in the electrical and electronic arts, with much less attention given to the other aspects of the systems. The mechanical arrangements are often quite rudimentary, though this would also be said of much of the rest of the "weight substitute" category. The result of this neglect of the mechanical arts has been a family of devices with desirable safety characteristics, but deficiencies in both performance and ergonomics.
The invention described in this application was created to rectify this situation through sophistication in mechanical design. Beginning with the excellent characteristics of the magnetic particle brake and the magnetic hysteresis brake, a set of specifications was established for the definitive weight substitute exercise apparatus; one in which there would be no significant compromise in performance, safety, or ergonomics.
The exercise apparatus of the present application described below was designed to accomplish the following objectives:
1. Resistance, as apparent to the user, must be completely "passive". The operating means must not move except under the impetus of the user.
2. Resistance, as apparent to the user, must vary only negligibly over the full travel of the operating means.
3. Resistance, as apparent to the user, must vary only negligibly with the speed of operation.
4. The operating means must move in a linear path, preferably vertically.
5.When the operating means is arranged to move vertically, its range of movement must be from below knee level to the "standing, both arms extended" level for normally proportioned people within a height range of 5' 0" to 6' 3". Movement within the range must be continuous.
6. Movement of the operating means should be bi-directional, with resistance for each direction being independently adjustable.
7. The range of adjustment for resistance should be at least 10:1 (e.g. 10 lbs. minimum and 100 lbs. maximum) with continuous adjustment between the limits.
8. Switching of resistances from one direction to the other should be automatic.
9. The apparatus should be such that no mechanical or electrical failure, or plausible combination of failures could cause the operating means to move without impetus from the user.
Meeting them required a variety of mechanical innovations, many of which are unprecedented in this field. A full scale, operating prototype has been constructed. Despite the rather makeshift construction typical in "proof of concept" prototypes, it meets the specifications in every respect.